Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

[Problems to be Solved] A neutron scintillator excellent in neutron
detection efficiency and n/γ discrimination ability, and a metal
fluoride eutectic preferred for the neutron scintillator are provided.
[Means to Solve the Problems] A metal fluoride eutectic having a
cerium-containing calcium fluoride crystal phase and a lithium fluoride
crystal phase present in a phase-separated state, and a neutron
scintillator comprising the metal fluoride eutectic.

Claims:

2. The metal fluoride eutectic according to claim 1, wherein both of the
crystal phases form a layered structure, and a thickness of each of
layers of the lithium fluoride crystal phase is 0.1 μm to 5 μm.

3. The metal fluoride eutectic according to claim 1, wherein the calcium
fluoride crystal phase is linearly continuous in one direction.

4. The metal fluoride eutectic according to claim 1, wherein a content of
cerium is 0.05 to 10 mol % based on calcium fluoride.

5. A neutron scintillator comprising the metal fluoride eutectic
according to claim 1.

Description:

TECHNICAL FIELD

[0001] This invention relates to a metal fluoride eutectic and a neutron
scintillator comprising the metal fluoride eutectic. More specifically,
the invention relates to a metal fluoride eutectic consisting
substantially of a lithium fluoride crystal phase and a calcium fluoride
crystal phase containing cerium.

BACKGROUND ART

[0002] Neutron detectors are a component technology supporting neutron
beam application technologies. With the progress of neutron beam
application technologies in academic research fields such as neutron
diffraction, in nondestructive inspection fields, or in security fields
such as cargo inspection, neutron detectors with higher performance are
desired.

[0003] Main performances demanded of the neutron detector are a detection
efficiency for neutrons and the count rate of neutrons, and the ability
to discriminate between neutrons and gamma rays (may hereinafter be
referred to as n/γ discrimination ability). The detection
efficiency means the ratio of the number of radiations counted by the
detector to the number of radiations emitted from a radiation source and
entered into the detector. The count rate means the number of radiations
counted per unit time. Gamma rays are generated when neutrons hit an
element contained in a constituent member of a detection system for
detecting neutrons, or in an object to be tested, such as Fe (iron), Pb
(lead), Cd (cadmium), C (carbon) or N (nitrogen). If the discrimination
ability for neutron beams versus gamma rays is low, a false signal due to
gamma ray which does not reflect the interaction between neutrons and the
object to be tested contaminate the true neutron signals, and a so-called
background noise increases.

[0004] Neutrons have a high power to pass through a substance without
doing any interaction in the substance. Therefore, a nuclear reaction for
promptly converting neutrons into charged particles having energy is
generally utilized to detect the neutron beam. For example, a helium-3
(3He) detector which detects neutrons by unitization of protons and
tritons generated by a nuclear reaction between 3He and neutrons has
so far been known. This detector has high detection efficiency and
excellent n/γ discrimination ability, but has posed the problem of
a limited count rate. Moreover, 3He is an expensive substance and
its resources are limited.

[0005] Recently, the development of a detector using a neutron
scintillator, instead of the above-mentioned 3He gas process, has
been underway in an attempt to produce an inexpensive and upsized
detector. The neutron scintillator refers to a substance which, when hit
by neutrons, absorbs the neutrons to emit fluorescence. The
aforementioned various performances of a neutron detector using this
scintillator depend on a substance constituting the scintillator. For
example, if an isotope, such as 6Li, which captures neutrons with
high efficiency is contained in a large amount in the substance
constituting the scintillator, the detection efficiency increases. If the
scintillator is composed of a light element which minimally interacts
with gamma rays, on the other hand, the background noise due to the gamma
rays is reduced. The decay time of fluorescence influences the count
rate.

[0006] LiF/ZnS has been used as a neutron scintillator having a relatively
high neutron detection efficiency and excellent n/γ discrimination
ability (see Non-Patent Document 1). Since the LiF/ZnS is opaque,
however, an increase in the thickness of the scintillator has made it
impossible to take out scintillation light efficiently. Thus, the LiF/ZnS
has been limited in the improvement of neutron detection efficiency.

[0007] In view of such problems, a proposal has been made for a neutron
scintillator comprising a eutectic composed of europium-containing
calcium fluoride crystals and lithium fluoride crystals (see Non-Patent
Document 2). This neutron scintillator composed of the eutectic is
translucent, and enables scintillation light to be collected with high
efficiency. Thus, this neutron scintillator can achieve a very high
neutron detection efficiency. According to studies by the inventors of
the present invention, however, the eutectic has been poor in n/γ
discrimination ability, and still has left room for improvement.

[0010] It is an object of the present invention to provide a neutron
scintillator excellent in neutron detection efficiency and n/γ
discrimination ability, and a eutectic substance preferred for the
neutron scintillator.

Means for Solving the Problems

[0011] The present inventors have conducted various studies on metal
fluoride eutectics composed of lithium fluoride crystals and calcium
fluoride crystals. As a result, they have found that a neutron
scintillator having excellent n/γ discrimination ability is
obtained by incorporating cerium into calcium fluoride crystals
constituting the metal fluoride eutectic. This finding has led them to
accomplish the present invention.

[0012] That is, according to the present invention, there is provided a
metal fluoride eutectic consisting substantially of a lithium fluoride
crystal phase and a calcium fluoride crystal phase containing cerium.

[0013] In the metal fluoride eutectic, it is preferred that

[0014] 1) the lithium fluoride crystal phase be of a layered structure
having layers each 0.1 μm to 5 μm thick;

[0015] 2) the content of cerium be 0.05 to 10 mol % based on calcium
fluoride; and

[0016] 3) the calcium fluoride crystal phase be linearly continuous in one
direction.

[0017] According to the present invention, there is also provided a
neutron scintillator comprising the metal fluoride eutectic.

Effects of the Invention

[0018] The present invention provides a neutron scintillator having high
neutron detection efficiency and excellent n/γ discrimination
ability, and a metal fluoride eutectic preferred for the neutron
scintillator. A neutron detector using such a neutron scintillator can be
preferably used in scientific research fields such as structural analyses
by neutron diffraction, nondestructive inspection fields, or security
fields such as cargo inspection.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] [FIG. 1] is a schematic view of the unidirectional solidification
process.

[0020] [FIG. 2] is a schematic view showing the layered structure of a
neutron scintillator according to the present invention.

[0021] [FIG. 3] is a compositional backscattered electron image of a cut
surface of a metal fluoride eutectic obtained in Example 1 when the metal
fluoride eutectic was cut in a direction parallel to a solidification
direction.

[0022] [FIG. 4] is a compositional backscattered electron image of a cut
surface of the metal fluoride eutectic obtained in Example 1 when the
metal fluoride eutectic was cut in a direction perpendicular to the
solidification direction.

[0030] The metal fluoride eutectic of the present invention is a metal
fluoride eutectic consisting substantially of two crystal phases, i.e.,
lithium fluoride (may hereinafter be referred to as LiF) crystals and
cerium-containing calcium fluoride (may hereinafter be referred to as
Ce:CaF2) crystals. This metal fluoride eutectic has a layered
structure in which the above two crystal phases are phase-separated.

[0031] The metal fluoride eutectic can be used preferably as a neutron
scintillator, because if emits scintillation light based on the following
course upon entry of neutrons. First, when neutrons are incident on LiF,
the neutrons are captured by 6Li-isotope in the LiF, whereby a
capture reaction takes place to generate secondary particles, i.e.,
α particles and tritium. Then, such secondary particles migrate in
the eutectic, reaching Ce:CaF2 and exciting the Ce:CaF2.
Finally, the excited Ce:CaF2 emits scintillation light. That is, the
LiF and Ce:CaF2 of the metal fluoride eutectic of the present
invention act, respectively, as a neutron capturing material and a
fluorescent substance emitting scintillation light.

[0032] In the above metal fluoride eutectic, the component ratio between
lithium fluoride and calcium fluoride (lithium fluoride/calcium fluoride)
is not limited. However, the preferred component ratio is 0.75 mol/0.25
mol to 0.85 mol/0.15 mol. Ideally, the component ratio at the eutectic
point for lithium fluoride and calcium fluoride, namely, lithium
fluoride/calcium fluoride=0.8 mol/0.2 mol, is particularly preferred. If
a molar ratio deviating greatly from such a component ratio at the
eutectic point is adopted, there is a possibility that the resulting
metal fluoride eutectic will be considerably cloudy, or that the
brightness of scintillation light will decline.

[0033] In the metal fluoride eutectic, it is preferred that the
6Li-isotope ratio of LiF be 20 to 99%. By setting the
6Li-isotope ratio at 20% or more, preferably 90% or more, the
probability for the capture reaction increases to raise the detection
efficiency for neutrons. In consideration of the cost involved in isotope
enrichment, on the other hand, the 6Li-isotope ratio is preferably
set at 99% or less.

[0034] The metal fluoride eutectic of the present invention is
characterized most greatly by containing Ce:CaF2 which functions as
the fluorescent substance emitting scintillation light. The brightness of
scintillation light from Ce:CaF2 is sufficiently high in the case of
excitation with secondary particles generated upon capture of neutrons,
but is specifically low in the case of excitation with gamma rays.
According to the metal fluoride eutectic of the present invention,
therefore, incidence of neutrons and incidence of gamma rays can be
easily discriminated based on the brightness of scintillation light, with
the result that a neutron scintillator excellent in n/γ
discrimination ability can be provided.

[0035] With the metal fluoride eutectic of the present invention, the
properties such as brightness of scintillation light vary with the
content of cerium in the calcium fluoride crystals. This content of
cerium can be adjusted by the amount of a cerium material added to a
material mixture when the metal fluoride eutectic is to be produced. The
cerium content is not limited, but is preferably set at 0.05 to 10 mol %
with respect to calcium fluoride. The cerium content of 0.05 mol % or
more can enhance the brightness of scintillation light, whereas the
cerium content of 10 mol % or less can avoid problems, such as marked
cloudiness of the metal fluoride eutectic or attenuation of scintillation
light due to concentration quenching. With a high cerium content, a
crystal phase of cerium fluoride may form aside from Ce:CaF2
crystals. Basically, however, the cerium fluoride crystal phase does not
present an obstacle, as long as the Ce:CaF2 crystals perform the
scintillation function by the aforementioned mechanism.

[0036] The exact state of existence of cerium atoms is unknown. In the
calcium fluoride crystals, however, most of the cerium atoms may be
present as substituted by Ca atoms, and some of them may be present
within a crystal lattice as well.

[0037] In the eutectic, the thickness of the laminar lithium fluoride
crystal layer is preferably 0.1 to 5 μm. As described above, secondary
particles produced upon capture of neutrons in the LiF crystals migrate
through the eutectic, and arrive at the Ce:CaF2 crystals. However,
their energy is partly lost during the course of their migration. When
the LiF crystal layer is thick, therefore, the energy imparted from the
secondary particles generated by the nuclear reaction to Ce:CaF2
varies greatly, thus leading to great variations in the brightness of
scintillation light emitted by Ce:CaF2. Investigations by the
present inventors have revealed that the thinner the LiF crystal layer,
the smaller variations the brightness of scintillation light shows, and
that by setting this thickness at 5 μm or less, a neutron scintillator
with excellent n/γ discrimination ability can be obtained. Setting
the thickness of the LiF crystal layer at less than 0.1 μm is
technically difficult, and requires special means. Thus, the lower limit
of such a thickness should be 0.1 μm.

[0038] Methods for producing the metal fluoride eutectic of the present
invention are not limited. Generally, there can be employed a method
which comprises mixing powders of the raw materials, i.e., lithium
fluoride, calcium fluoride and cerium fluoride, to prepare a material
mixture, heating the material mixture to melt it, and then cooling the
resulting melt for solidification. However, this method is not
necessarily preferred in incorporating a relatively large amount of
cerium into calcium fluoride crystals, and may decrease the brightness of
scintillation light. This phenomenon is specifically observed when a
trivalent lanthanoid such as cerium is added to a metal fluoride eutectic
composed of lithium fluoride and calcium fluoride, and the cause of this
phenomenon is uncertain.

[0039] According to investigations by the present inventors, the following
method of production has been found preferred in incorporating a large
amount of cerium: the method which comprises: separately preparing a
material comprising calcium fluoride crystals containing cerium
(hereinafter referred to as a Ce:CaF2 material); mixing the
Ce:CaF2 material and lithium fluoride to prepare a material mixture;
heating the material mixture to melt it; and then cooling the resulting
melt for solidification. According to this production method, a
sufficient amount of cerium can be incorporated into calcium fluoride
crystals, and thus the brightness of scintillation light can be enhanced
sufficiently.

[0040] A method for preparing the Ce:CaF2 material is not limited.
However, the preferred method comprises mixing a powder of cerium
fluoride and a powder of calcium fluoride to prepare a material mixture,
heating the material mixture to melt it, and then cooling the resulting
melt for solidification. The amount of cerium fluoride added is not
limited, but its amount of 0.05 to 10 mol % based on calcium fluoride
results in the aforementioned cerium content in the calcium fluoride
crystals.

[0041] Other investigations by the present inventors have shown the
following facts: In the above-mentioned production method comprising
melting a material mixture of powders of the respective materials, and
solidifying the resulting melt, the addition, as an additive, of at least
one alkali metal fluoride selected from NaF, KF, RbF and CsF is preferred
particularly when a large amount of cerium is incorporated. Also in
directly producing the metal fluoride eutectic from the respective
materials, lithium fluoride, calcium fluoride and cerium fluoride, the
addition of such an additive enables a sufficient amount of cerium to be
incorporated in calcium fluoride crystals. Compared with the above method
using the Ce:CaF2 material, moreover, such a direct production
method can simplify the manufacturing process, and can thus reduce the
manufacturing cost markedly.

[0042] As the alkali metal fluoride, sodium fluoride or potassium fluoride
is preferred. The amount of the alkali metal fluoride added is not
limited, but is preferably 10 to 1000 mol % based on cerium fluoride, and
0.05 to 10 mol % based on calcium fluoride. By setting the amount of the
alkali metal fluoride added at 10 mol % or more based on cerium fluoride
and 0.05 mol % or more based on calcium fluoride, cerium can be contained
efficiently in the calcium fluoride crystals. An extreme increase in its
amount added, on the other hand, may separately form a crystal phase of
the alkali metal fluoride, thereby causing marked cloudiness of the
resulting metal fluoride eutectic. Thus, the amount of the alkali metal
fluoride added is preferably 1000 mol % or less based on cerium fluoride,
and 10 mol % or less based on calcium fluoride.

[0043] The method for producing the metal fluoride eutectic of the present
invention will now be described.

[0044] First, powders of the materials, lithium fluoride, calcium fluoride
and cerium fluoride, are mixed to prepare a material mixture. When the
manufacturing method using the Ce:CaF2 material is adopted, however,
lithium fluoride and the Ce:CaF2 material are mixed to prepare a
material mixture. Also, the aforementioned alkali metal fluoride may be
added as an additive to the material mixture.

[0045] In the present invention, the purities of the materials, lithium
fluoride, calcium, fluoride and cerium fluoride, and the alkali metal
fluoride as the additive are not limited, but are preferably 99.99% or
higher. By using such high purity materials and additive, the properties
such as the brightness of scintillation light are improved. The materials
and the additive may be used in powdery or particulate form, or may be
sintered or melt-solidified beforehand and then used.

[0046] The material mixture is charged into a crucible, and heated to be
melted. Then, a melt of the material mixture melted is cooled to be
solidified. During the solidification, phase separation between an LiF
phase and a Ce:CaF2 phase takes place, and a metal fluoride eutectic
is formed simultaneously with the solidification. In order to control a
layered structure (thickness, continuity, linearity), in particular, the
unidirectional solidification process for solidifying the melt in one
particular direction can be used preferably.

[0047] The unidirectional solidification process will be illustrated by
FIG. 1 taken as an example. A melt is placed inside a furnace body
adjusted to a high temperature in its upper portion and a low temperature
in its lower portion. When either the melt is lowered, or the furnace
body is raised, the melt is cooled from its lower part, and a part of the
melt below the solidification point solidifies to become a metal fluoride
eutectic. At this time, a solid-liquid interface is formed between the
metal fluoride eutectic and the melt. When the melt is further lowered,
or the furnace body is further raised, the solid-liquid interface moves
upward and the metal fluoride eutectic extends. By performing such a
procedure continuously, the melt can be solidified in one specific
direction. In the present invention, the direction in which the melt is
solidified in one specific direction is referred to as a solidification
direction, and a rate at which solidification is allowed to proceed is
referred to as a solidification rate.

[0048] According to the above-mentioned unidirectional solidification
process, the thickness of the LiF crystal layer in the metal fluoride
eutectic can be easily decreased by increasing the solidification rate,
and a metal fluoride eutectic having a thickness of the LiF crystal layer
of 5 μm or less can be produced efficiently. By means of the
unidirectional solidification process, moreover, Ce:CaF2 crystals as
well as LiF crystals extend along the solidification direction as shown
in FIG. 1. As a result, a metal fluoride eutectic having the LiF crystals
linearly continuous in one direction and also the Ce:CaF2 crystals
linearly continuous in the one direction can be produced.

[0049] The metal fluoride eutectic having the Ce:CaF2 crystals
linearly continuous in one direction as above exhibits special effects
when combined with a photodetector to construct a neutron detector. That
is, in a eutectic composed of LiF crystals and Ce:CaF2 crystals, the
Ce:CaF2 crystals generally fail to be linearly continuous in one
direction. When scintillation light is emitted from Ce:CaF2,
therefore, the scintillation light is scattered at the interface between
the Ce:CaF2 crystals and the LiF crystals. As a result, the
propagation efficiency of scintillation light toward the photodetector
decreases. In a neutron scintillator using a metal fluoride eutectic in
which Ce:CaF2 crystals are linearly continuous in one direction, by
contrast, the propagation efficiency of scintillation light is high in
the direction of linear continuity of the Ce:CaF2 crystals (the
direction indicated by an arrow in FIG. 2). Thus, a photodetector is
placed at an end in the direction of the continuous crystals, whereby the
detection efficiency of scintillation light in the photodetector can be
increased.

[0050] The thickness of the Ce:CaF2 crystal layer is not limited,
because it does not affect the propagation efficiency of scintillation
light. However, the thickness is, for example, 0.1 to 10 μm.

[0051] Concrete methods illustrating the unidirectional solidification
process are: the Bridgman method in which a crucible charged with a melt
is placed within a furnace body adjusted to a high temperature in an
upper portion thereof and a low temperature in a lower portion thereof,
and the crucible is lowered to solidify the melt in one direction from
bottom to top; the gradient freeze method in which a crucible charged
with a melt is placed within a furnace body adjusted to a high
temperature in an upper portion thereof and a low temperature in a lower
portion thereof, and cooling is performed, with a temperature
distribution being kept to hold the lower portion always at a low
temperature, whereby the melt is solidified in one direction from bottom
to top; the Czochralski method in which with a solid-liquid interface
being held at a constant position, a metal fluoride eutectic is pulled up
while being solidified in one direction; and the micro-pulling-down
method in which with a solid-liquid interface being held at a constant
position, a metal fluoride eutectic is pulled down while being solidified
in one direction.

[0052] The production method adopting the unidirectional solidification
process will be described in detail, with the Bridgman method taken as an
example.

[0053] First, powders of lithium fluoride, calcium fluoride and cerium
fluoride as starting materials are mixed to prepare a material mixture.
If the aforementioned alkali metal fluoride is added, however, the
material mixture having the alkali metal fluoride added thereto is used
as the starting material. If the method of preparing the Ce:CaF2
material separately beforehand is adopted, a powder or the like of
lithium fluoride and the Ce:CaF2 material are mixed to prepare a
material mixture.

[0054] Then, the material mixture is charged into a crucible, and the
charged crucible is set in a chamber equipped with a heater, a heat
insulator, and a vacuum evacuator. Using the vacuum evacuator, the
interior of the chamber is evacuated to 1.0×10-3 Pa or lower.
Then, an inert gas such as high purity argon is introduced into the
chamber for a gas exchange operation. The pressure within the chamber
after the gas exchange operation is not limited, but is generally
atmospheric pressure. By this gas exchange operation, water adhering to
the starting materials or the interior of the chamber can be removed.
Consequently, problems such as attenuation of scintillation light of the
metal fluoride eutectic, which is ascribed to such water, can be avoided.

[0055] To avoid adverse influence due to water which cannot be removed
even by the above gas exchange operation, it is preferred to remove water
with the use of a scavenger highly reactive with water. As such a
scavenger, a gaseous scavenger such as tetrafluoromethane can be used
preferably. When the gaseous scavenger is used, the preferred method is
to mix it with the above-mentioned inert gas and introduce the mixture
into the chamber.

[0056] After the gas exchange operation is performed, the material mixture
is heated by the heater until it is melted. The temperature when melting
the material mixture differs according to the chemical composition of the
material mixture, but is generally in the range of 770° C. to
900° C. which is the eutectic point of lithium fluoride and
calcium fluoride. The method of heating by the heater is not limited, and
a high frequency induction heating method or a resistance heating method,
for example, can be used as appropriate.

[0057] Then, a melt of the molten material mixture is lowered together
with the crucible. Since the heater and the heat insulator are arranged
so as to be at a high temperature in their upper parts and at a low
temperature in their lower parts, the melt solidifies, beginning in its
lower portion, as it descends. During such solidification, phase
separation between LiF crystals and Ce:CaF2 crystals occurs, and a
metal fluoride eutectic forms simultaneously with solidification. By
further lowering the melt continuously, the melt solidifies
unidirectionally from bottom to top, and the metal fluoride eutectic
extends along the solidification direction. Thus, a metal fluoride
eutectic having Ce:CaF2 crystal layers linearly continuous in one
direction can be produced.

[0058] In the above-described Bridgman method, the rate at which to lower
the melt, namely, the solidification rate, is not limited, but is
preferably 2 to 50 mm/hr. The faster the solidification rate, the thinner
the LiF crystal layer becomes. By setting the solidification rate at 2
mm/hr or higher, a metal fluoride eutectic with the LiF crystal layer
thickness of 5 μm or less can be produced. If the solidification rate
exceeds 50 mm/hr, on the other hand, cloudiness or cracking of the metal
fluoride eutectic may be noticeable. Thus, the solidification rate is
preferably set at 50 mm/hr or lower.

[0059] A temperature change per unit distance along the solidification
direction, namely, a temperature gradient, is not limited, but is
preferably 0.5° C./mm or more. By setting the temperature gradient
at 0.5° C./mm or more, the unidirectionality of the Ce:CaF2
crystal layers can be enhanced. The upper limit of the temperature
gradient is not limited, but is generally 10° C./mm or less.

[0060] The layered structure of the resulting metal fluoride eutectic can
be identified by observing a compositional backscattered electron image
with the use of a scanning electron microscope (SEM). That is, in the
compositional backscattered electron image, the LiF crystal and the
CaF2 crystal exhibit a distinct contrast to each other based on the
difference between their atomic numbers, so that an image reflecting the
layered structure as shown in FIG. 1 can be easily obtained.

[0061] The identification of the crystal phases constituting the metal
fluoride eutectic can be made by powder X-ray diffraction measurement.
That is, by making the powder X-ray diffraction measurement of a powder
formed by pulverizing the metal fluoride eutectic and analyzing a
diffraction pattern obtained, the metal fluoride eutectic is identified
as a metal fluoride eutectic composed of LiF crystals and CaF2
crystals.

[0062] The metal fluoride eutectic of the present invention has
satisfactory processability, and is easily used as processed into a
desired shape. In processing the metal fluoride eutectic, a cutter such
as a blade saw or a wire saw, a grinder or an abrasive wheel, which is
publicly known, can be used without limitation.

[0063] The metal fluoride eutectic of the present invention can be
processed into a desired shape to form a neutron scintillator according
to the present invention. The shape of the neutron scintillator may be
any shape, including the shape of a plate, a block, or an array of a
plurality of quadrangular prism-shaped metal fluoride eutectics arranged.
Moreover, the neutron scintillator comprising the metal fluoride eutectic
of the present invention is combined with a photo-detector such as a
photomultiplier tube, whereby a neutron detector can be constituted. With
this neutron detector, scintillation light emitted from the neutron
scintillator upon irradiation with neutrons is converted into an
electrical signal by the photomultiplier tube, whereby the presence or
absence and strength of neutrons can be grasped as the electrical signal.
Scintillation light emitted by the neutron scintillator of the present
invention is light with a wavelength of about 300 to 350 nm, and the
photomultiplier tube capable of detecting light in this region can be
used particularly preferably. Specific examples of such a photomultiplier
tube are R7600U and H7416 produced by Hamamatsu Photonics K.K.

[0064] Concretely, there can be named, for example, a method which
comprises coupling the neutron scintillator of the present invention to
the photoelectric surface of the photomultiplier tube with the use of an
optical grease or the like, applying a high voltage to the
photomultiplier tube, and measuring an electrical signal outputted by the
photomultiplier tube. In order to analyze the strength of a neutron beam
or the like by utilizing the electrical signal from the photomultiplier
tube, an amplifier, a multi-channel analyzer or the like may be provided
at a stage subsequent to the photomultiplier tube. Furthermore, the
neutron scintillator composed of the metal fluoride eutectic according to
the present invention can be combined with a position-sensitive
photodetector to construct a neutron imaging device. As the
position-sensitive photodetector, a position-sensitive photomultiplier
tube can be preferably used. Its examples include XP85012 produced by
PHOTONIS USA INC.

EXAMPLES

[0065] Hereinbelow, the present invention will be described specifically
by reference to its Examples, but the present invention is in no way
limited by these Examples. Moreover, not all of combinations of the
features described in the Examples are essential to the means for
solution to problems that the present invention adopts.

Example 1

Preparation of Ce:CaF2 Material

[0066] First, 2.3 g of cerium fluoride was added to 180 g of calcium
fluoride, i.e., cerium was added in a proportion of 0.5 mol % with
respect to calcium fluoride, followed by thorough mixing, and charging
the mixture into a crucible. The calcium fluoride and the cerium fluoride
used were powders with purities of 99.99% or higher.

[0067] Then, the crucible charged with the calcium fluoride and cerium
fluoride was set in a chamber equipped with a vacuum evacuator and a
heater. Using the vacuum evacuator, the interior of the chamber was
evacuated to 5.0×10-4 Pa or lower. Then, a mixed gas
consisting of 5 vol. % tetrafluoromethane and 95 vol. % high purity argon
was introduced into the chamber for a gas exchange operation.

[0068] After the gas exchange operation, the above calcium fluoride and
cerium fluoride were heated to 1500° C. using a resistance heating
type carbon heater until they were melted. Then, the melt was cooled and
solidified to obtain a Ce:CaF2 material.

Production of Metal Fluoride Eutectic

[0069] First, 180 g of the above Ce:CaF2 material was chipped, and
230 g of lithium fluoride was added, whereafter these materials were
thoroughly mixed to prepare a material mixture. As the lithium fluoride,
a powder having a 6Li-isotope ratio of 95% and purity of 99.99% or
more was used. The mixing ratio between lithium fluoride and calcium
fluoride in the material mixture was 0.8 lithium fluoride/0.2 calcium
fluoride (mol/mol), and the amount of cerium added was 0.5 mol % with
respect to calcium fluoride.

[0070] Then, the material mixture was charged into a crucible formed of
carbon and having an internal diameter of 50 mm, and the crucible was set
in a chamber equipped with a resistance heating type heater, a heat
insulator, and a vacuum evacuator. Using the vacuum evacuator, the
interior of the chamber was evacuated to 2.0×10-4 Pa or lower.
Then, a high purity argon gas mixed with 5 vol. % tetrafluoromethane was
introduced into the chamber for a gas exchange operation. The pressure
within the chamber after the gas exchange operation was atmospheric
pressure.

[0071] After the gas exchange operation was performed, the material
mixture was heated by the heater until it was melted.

[0072] The heater and the heat insulator were arranged such that a
temperature gradient in the solidification direction was 2.5°
C./mm, and the output of the heater was adjusted such that the
temperature at the bottom of the crucible was 830° C.

[0073] Then, the melt of the material mixture melted was continuously
lowered together with the crucible to solidify the melt unidirectionally
from bottom to top. In the present Example, the rate at which the melt
was lowered, namely, the solidification rate, was set at 10 mm/hr. By
this procedure, the melt was totally solidified to obtain a metal
fluoride eutectic used in the present invention.

[0074] The resulting metal fluoride eutectic was cut in a direction
parallel to the solidification direction end in a direction perpendicular
to the solidification direction by a wire saw provided with a diamond
wire, and the cut surface was mirror polished, a compositional
backscattered electron image of the surface when cut in the direction
parallel to the solidification direction is shown in FIG. 3. A
compositional backscattered electron image of the surface when cut in the
direction perpendicular to the solidification direction is shown in FIG.
4. Highlights of the compositional backscattered electron images
represent CaF2 crystal layers, while shadows of the compositional
backscattered electron images represent LiF crystal layers.

[0075] FIG. 3 shows that the CaF2 crystal layers were linearly
continuous in one direction. In connection with the compositional
backscattered electron image of FIG. 4, the thickness of the LiF crystal
layer was measured using the length measuring function of the SEM. The
thickness of the layer of LiF in the metal fluoride eutectic was found to
be 3 μ. In measuring the thickness of the LiF crystal layer,
calibration was performed using a standard grid with a spacing length of
23 μm to carry out the measurement.

[0076] The resulting metal fluoride eutectic was pulverized to form a
powder, which was subjected to powder X-ray diffraction measurement,
whereby the crystal phases were identified. "D8 DISCOVER" produced by
Bruker AXS K.K was used as a measuring device. A diffraction pattern
obtained is shown in FIG. 5. From FIG. 5, diffraction peaks attributed to
LiF crystals and CaF2 crystals were confirmed, showing that the
metal fluoride eutectic was a metal fluoride eutectic composed of LiF
crystals and CaF2 crystals.

Preparation and Characteristics Evaluation of Neutron Scintillator

[0077] The resulting metal fluoride eutectic was cut by a wire saw
provided with a diamond wire, and was then ground and mirror polished to
be processed into a shape 7 mm in length, 2 mm in width, and 1 mm in
thickness. In this manner, a neutron scintillator according to the
present invention was obtained. The characteristics of the neutron
scintillator in response to neutrons were evaluated by the following
method:

[0078] The surface of the neutron scintillator, measuring 7 mm in length
and 2 mm in width, was coupled to the photoelectric surface of the
photomultiplier tube (R7600U produced by Hamamatsu Photonics K.K.) with
the use of an optical grease to produce a neutron detector.

[0079] The neutron detector was covered with a light shielding material
made of a black vinyl sheet so that external light would not enter the
photoelectric surface of the photomultiplier tube. Then, the neutron
scintillator was irradiated with neutrons from 252Cf with
radioactivity of 1 MBq, the neutrons being slowed down via a neutron
moderator comprising a polyethylene block having a thickness of 40 mm. To
measure scintillation light emitted from the neutron scintillator, a high
voltage of 800V was applied from a power supply line to the
photomultiplier tube, whereby scintillation light was converted into an
electrical signal, which was outputted from a signal output line. The
electrical signal outputted from the photomultiplier tube is a pulsed
signal reflecting the scintillation light, and the pulse height of the
pulsed signal represents the brightness of scintillation light. Such
electrical signals outputted from the photomultiplier tube were shaped
and amplified by a shaping amplifier, and then entered into a
multichannel pulse height analyzer to analyze them and prepare a pulse
height spectrum.

[0080] The resulting pulse height spectrum is shown in FIG. 6. The
abscissa of the pulse height spectrum represents the pulse height value
of the electrical signal, namely, the brightness of the scintillation
light, indicating here the value of the pulse height channel of the
multichannel pulse height analyzer. The ordinate represents the frequency
of the electrical signal showing each pulse height value. The frequency
was expressed as the number of times (counts) the electrical signal
concerned was measured.

[0081] Clear peaks indicating the detection of neutrons were confirmed
from FIG. 6. This finding has proved that the metal fluoride eutectic of
the present invention acts effectively as a neutron scintillator, and
that the neutron detector using the neutron scintillator is effective.

[0082] To evaluate the n/γ discrimination ability of the neutron
scintillator, scintillation light emitted upon excitation with alpha rays
and scintillation light emitted upon excitation with gamma rays were
measured and compared with each other by the following methods:

[0083] In the case of excitation with alpha rays, 241Am having
radioactivity of 4 MBq was placed in proximity to the neutron
scintillator of the neutron detector, and the neutron detector was
shielded from light with a light shielding sheet so that external light
would not enter under irradiation with alpha rays. In the case of
excitation with gamma rays, after shielding from light with a light
shielding sheet so that external light would not enter, 137Cs having
radioactivity of 1 kBq was placed at a position about 30 mm apart from
the neutron scintillator, and the neutron scintillator was irradiated
with gamma rays.

[0084] To measure scintillation light emitted when the neutron
scintillator was excited with each of alpha rays and gamma rays, the
scintillation light was converted into an electrical signal via a
photomultiplier tube to which a high voltage of 800V was applied. Such
electrical signals outputted from the photomultiplier tube were shaped
and amplified by a shaping amplifier, and then entered into a
multichannel pulse height analyzer to analyze them and prepare a pulse
height spectrum.

[0085] The resulting pulse height spectra are shown in FIG. 7. Solid lines
and dotted lines, respectively, represent the pulse height spectra
obtained by excitation with alpha rays and gamma rays. The abscissa
represents the pulse height value expressed as a relative value with
respect to the peak value of the pulse height spectrum, which is obtained
upon excitation with alpha rays, defined as 1.

[0086] In the resulting pulse height spectrum, the pulse height value
obtained when the neutron scintillator was excited with gamma rays was
0.3 at the maximum, a value sufficiently low as compared with the peaks
of the pulse height values in the case of alpha ray excitation. It is
seen, therefore, that gamma-ray noises can be easily eliminated, for
example, by taking the pulse height value indicated by the chain line of
FIG. 7 as a threshold value, removing electrical signals of pulse height
values below this threshold value, and selecting only electrical signals
of pulse height values exceeding the threshold value. That is, the
neutron scintillator of the present invention has been found to have
excellent n/γ discrimination ability.

Example 2

Production of Metal Fluoride Eutectic

[0087] First, 180 g of calcium fluoride, 230 g of lithium fluoride, 2.3 g
of cerium fluoride, end 1.4 g of sodium fluoride as an additive were
weighed, and thoroughly mixed to prepare a material mixture. The
respective materials and the additive sodium fluoride used were powders
with purities of 99.99% or higher, and the 6Li-isotope ratio of the
lithium fluoride was 95%.

[0088] In the material mixture, the mixing ratio between lithium fluoride
and calcium fluoride was 0.8 lithium fluoride/0.2 calcium fluoride
(mol/mol), and the amount of cerium added was 0.5 mol % with respect to
calcium fluoride. The amount of the additive sodium fluoride added was
300 mol % with respect to cerium fluoride and 1.5 mol % with respect to
calcium fluoride.

[0089] Then, the material mixture was charged into a crucible formed of
carbon and having an internal diameter of 50 mm, and the crucible was set
in a chamber equipped with a resistance heating type heater, a heat
insulator, and a vacuum evacuator. Using the vacuum evacuator, the
interior of the chamber was evacuated to 2.0×10-4 Pa or lower.
Then, a high purity argon gas mixed with 5 vol. % tetrafluoromethane was
introduced into the chamber for a gas exchange operation. The pressure
within the chamber after the gas exchange operation was atmospheric
pressure.

[0090] After the gas exchange operation was performed, the material
mixture was heated by the heater until it was melted.

[0091] The heater and the heat insulator were arranged such that a
temperature gradient in the solidification direction was 2.5°
C./mm, and the output of the heater was adjusted such that the
temperature at the bottom of the crucible was 830° C.

[0092] Then, the melt of the material mixture melted was continuously
lowered together with the crucible to solidify the melt unidirectionally
from bottom to top. In the present Example, the rate at which the melt
was lowered, namely, the solidification rate, was set at 10 mm/hr. By
this procedure, all of the melt was solidified to obtain a metal fluoride
eutectic used in the present invention.

[0093] The resulting metal fluoride eutectic was processed in the same
manner as in Example 1, and its compositional backscattered electron
image was observed. In the metal fluoride eutectic of the present
Example, it was found that the CaF2 crystal layers were linearly
continuous in one direction, and the thickness of the layer of LiF was 3
μm.

[0094] A powder of the resulting metal fluoride eutectic pulverized was
subjected to powder X-ray diffraction measurement in the same manner as
in Example 1, whereby the crystal phases were identified. A diffraction
pattern obtained is shown in FIG. 5. From FIG. 5, diffraction peaks
attributed to LiF crystals and CaF2 crystals were confirmed, showing
that the metal fluoride eutectic was a metal fluoride eutectic composed
of LiF crystals and CaF2 crystals.

Preparation and Characteristics Evaluation of Neutron Scintillator

[0095] The resulting metal fluoride eutectic was processed in the same
manner as in Example 1 to obtain a neutron scintillator according to the
present invention. The characteristics of the neutron scintillator in
response to neutrons were evaluated in the same manner as in Example 1. A
pulse height spectrum obtained is shown in FIG. 8.

[0096] Clear peaks indicating the detection of neutrons were confirmed
from FIG. 8. This finding has proved that the metal fluoride eutectic of
the present invention acts effectively as a neutron scintillator, and
that a neutron detector using the neutron scintillator is effective.

[0097] To evaluate the n/γ discrimination ability of the neutron
scintillator, scintillation light emitted upon excitation with alpha rays
and scintillation light emitted upon excitation with gamma rays were
measured and compared in the same manner as in Example 1. Pulse height
spectra obtained are shown in FIG. 9. In the resulting pulse height
spectra, the pulse height value obtained by excitation with gamma rays
was 0.35 at the maximum, which was sufficiently low as compared with the
peaks of the pulse height values obtained upon alpha ray excitation.
Thus, the neutron scintillator of the present invention has been found to
have excellent n/γ discrimination ability.

[0099] First, 180 g of calcium fluoride, 230 g of lithium fluoride, and
2.4 g of europium fluoride were weighed, and thoroughly mixed to prepare
a material mixture. The respective materials used were powders with
purities of 99.99% or higher, and the 6Li-isotope ratio of the
lithium fluoride was 95%.

[0100] In the material mixture, the mixing ratio between lithium fluoride
and calcium fluoride was 0.8 lithium fluoride/0.2 calcium fluoride
(mol/mol), and the amount of europium added was 0.5 mol % with respect to
calcium fluoride.

[0101] Then, the material mixture was charged into a crucible formed of
carbon and having an internal diameter of 50 mm, and the crucible was set
in a chamber equipped with a resistance heating type heater, a heat
insulator, and a vacuum evacuator. Using the vacuum evacuator, the
interior of the chamber was evacuated to 2.0×10-4 Pa or lower.
Then, a high purity argon gas mixed with 5 vol. % tetrafluoromethane was
introduced into the chamber for a gas exchange operation. The pressure
within the chamber after the gas exchange operation was atmospheric
pressure.

[0102] After the gas exchange operation was performed, the material
mixture was heated by the heater until it was melted.

[0103] The heater and the heat insulator were arranged such that a
temperature gradient in the solidification direction was 2.5° C.
/mm, and the output of the heater was adjusted such that the temperature
at the bottom of the crucible was 830° C.

[0104] Then, the melt of the material mixture melted was continuously
lowered together with the crucible to solidify the melt unidirectionally
from bottom to top. In the present Example, the rate at which the melt
was lowered, namely, the solidification rate, was set at 10 mm/hr. By
this procedure, all of the melt was solidified to obtain a metal fluoride
eutectic.

[0105] The resulting metal fluoride eutectic was processed in the same
manner as in Example 1, and its compositional backscattered electron
image was observed. In the eutectic of the present Comparative Example,
it was found that the CaF2 crystal layers were linearly continuous
in one direction, and the thickness of the layer of LiF was 3 μm.

[0106] A powder of the resulting metal fluoride eutectic pulverized was
subjected to powder X-ray diffraction measurement in the same manner as
in Example 1, whereby the crystal phases were identified. A diffraction
pattern obtained is shown in FIG. 5. From FIG. 5, diffraction peaks
attributed to LiF crystals and CaF2 crystals were confirmed, showing
that the metal fluoride eutectic was a metal fluoride eutectic composed
of LiF crystals and CaF2 crystals.

Preparation and Characteristics Evaluation of Neutron Scintillator

[0107] The resulting metal fluoride eutectic was processed in the same
manner as in Example 1 to obtain a neutron scintillator. The
characteristics of the neutron scintillator in response to neutrons were
evaluated in the same manner as in Example 1. A pulse height spectrum
obtained is shown in FIG. 10.

[0108] Clear peaks indicating the detection of neutrons were confirmed
from FIG. 10. This finding has proved that the metal fluoride eutectic of
the present Comparative Example acts as a neutron scintillator. To
evaluate the n/γ discrimination ability of the neutron
scintillator, scintillation light emitted upon excitation with alpha rays
and scintillation light emitted upon excitation with gamma rays were
measured and compared in the same manner as in Example 1.

[0109] Pulse height spectra obtained are shown in FIG. 11. In the
resulting pulse height spectra, the pulse height value obtained when
excited with gamma rays reached 0.7 at the maximum. Thus, the metal
fluoride eutectic composed of lithium fluoride crystals and
europium-containing calcium fluoride crystals has been found to be poor
in n/γ discrimination ability when applied as a neutron
scintillator. A comparison between the neutron scintillator of the
present Comparative Example and the neutron scintillators of the present
invention shown in Examples 1 and 2 has revealed that the neutron
scintillator comprising the metal fluoride eutectic according to the
present invention has particularly excellent n/γ discrimination
ability in comparison with the neutron scintillators comprising metal
fluoride eutectics which have been known publicly.

Patent applications by Akira Yoshikawa, Sendai-Shi JP

Patent applications by Kentaro Fukuda, Shunan-Shi JP

Patent applications by Noriaki Kawaguchi, Shunan-Shi JP

Patent applications by Takayuki Yanagida, Sendai-Shi JP

Patent applications by Yui Yokota, Sendai-Shi JP

Patent applications by TOHOKU UNIVERSITY

Patent applications by TOKUYAMA CORPORATION

Patent applications in class No layer or component greater than 5 mils thick

Patent applications in all subclasses No layer or component greater than 5 mils thick